Firefighting foam concentrates are mixtures of foaming agents, solvents and other additives. These concentrates are intended to be mixed with water, the resulting solution foamed by mechanical means, and the foam projected onto the surface of a burning liquid. A particular class of firefighting foam concentrates is known as an aqueous film-forming foam (AFFF). AFFF concentrates have the quality of being able to spread an aqueous film on the surface of hydrocarbon liquids, enhancing the speed of extinguishment of fuel and solvent fires. Surfactants added to AFFF lower surface tension values which permits the foam to spread on the surface of the hydrocarbon liquids.
Aqueous film-forming foams provide a blanket to cover the fuel surface excluding air preventing further ignition of the fuel. For this reason aqueous film-forming foam compositions are particularly desirable for extinguishing fires involving flammable fuels, such as gasoline, naphtha, diesel oils, hydraulic fluids and other hydrocarbons.
Aqueous film-forming foams need a surfactant to impart important film forming properties that are useful in the extinguishment of burning liquids. Alkyl polyglycosides (APG's) are a well known class of non-ionic surfactants. APG's may be prepared by acid catalyzed reaction of fatty alcohols with a mono- or disaccharide (e.g. glucose, galactose, sucrose, maltose, etc.), or a polysaccharide source of these sugars (e.g. starch or corn syrup). A variety of reaction conditions for synthesizing APG's are known using various starting materials and various types of acid catalysts.
Schulz and Flory developed a one-parameter equation to represent the distribution of products in a polymer reaction which is applicable to the linear condensation polymerization of monosaccharides to form di- or polysaccharides. Although it is possible to control the ratio of products to some extent by adjusting the stoichiometry of reactants, the Schulz and Flory equation predicts that the monoglycoside predominates. Typically, the resulting product is composed of around 50 to 70% of the monoglycoside. However, the product also contains decreasing amounts of di-, tri-, tetra-, etc. glycosides, which is referred to as the “Flory distribution” of mono and polyglycosides. (U.S. Pat. No. 5,962,399) Thus, the “Flory distribution” refers to a product mixture of predominantly monoglycoside and decreasing amounts of each higher polyglycoside. For example, the disaccharide would be present in a molar amount less than the monosaccharide, but more than the trisaccharide, etc.
It has been suggested that the polyglycoside portion of the product mixture has superior surfactant properties over the monoglycoside (U.S. Pat. No. 3,598,865), and several methods have been proposed for enriching the polyglycoside portion of the product by removing monoglycoside by solvent extraction (U.S. Pat. Nos. 3,219,656; 3,547,828; and Talley et al., J. Am. Chem. Soc., 1945, 67, 2037-2039 or vacuum distillation (U.S. Pat. No. 5,962,399).
Fluorine analogs of the APG's are also known in which the alkyl chain is partly or completely perfluorinated. These materials have found use as liquid crystals (Miethchen & Hein, Carbohydrate Research 2000, 327, 169-183) and biomedical emulsifying agents (Riess & Greiner, Carbohydrate Research 2000, 327, 147-168; U.S. Pat. No. 4,985,550).
Among the variety of preparative methods disclosed in these publications are specific procedures for the synthesis of protected mono- and di-glycosides using Koenigs-Knorr (Riess et al., New J. Chem. 1991, 15, 337-344) or Mitsunobu chemistry (Rollin et al., Carbohydrate Research 1999, 318, 171-179), or by radical addition of perfluoroalkyl iodides to alkenyl glycosides such as allyl glucoside (Miethchen & Hein, Tet. Letters 1998, 39, 6679-6682) or pentenyl maltoside (Riess et al., New J. Chem. 1991, 15, 337-344).
Radical addition chemistry allows a variety of perfluoroalkyl groups to be added to a common sugar derivative, and a wide variety of such sugar derivatives are accessible using known carbohydrate chemistry. This chemistry generally involves a number of synthetic steps including protection-deprotection of the specific sugar being derivatized (Riess et al., New J. Chem. 1991, 15, 337-344; Riess et al., J. Med. Chem. 1990, 33, 1262-1269; Huang, J. Fluorine Chem. 1992, 58, 1-8; Yuasa & Yuasa, Org. Process Research & Development 2004, 8, 405-407). A chemical process for the preparation of perfluoroalkyl thioglycosides for biomedical and surfactant use is also known (U.S. Pat. No. 4,957,904).
What is needed are nonionic fluoroalkyl glycosides which are enriched in the polyglycoside which can enhance the fire extinguishment properties and stability of firefighting foams.